I'd take a different approach to heaters. Most of us on here have an STC-1000 temperature controller (make sure it the version with the rubber probe, not a metal probe, as they rust). They are very accurate (+/- 0.3 degrees), and you can plug in two heaters (in case one stops working) both of which should be able to heat the tank on their own - too small and if one stops working your tank can get too cold in winter. You can then set the thermostat on both to click off just past the point the STC would switch them off (just incase the STC fails - not heard of this happening yet). If a heater gets stuck on, the STC turns it off anyway. You'd be VERY unlucky to have a heater stick on, and the STC fail.